... interesting question....

I'll take the Hitler-example due to my lack of knowledge about the American Revolution.....

First: a man like Hitler was needed to lead Germany into WWII. There were a lot of right-winged parties and organisations in the Republic of Weimar and the Hitler-Ludendorff-Putsch in 1923 was not the only one from this side. But most of these factions wanted to have monarchy back of fight against communism or something like that. But I can't think of any organisation proclaiming that clear to exterminate judaism, communism and pacifism (of the so called 'November criminals'). And this was Hitler's influence. He was so possessed by these wishes, that he lost his ability to judge. Nearly every member of his party would have been more careful in foreign politics but he wanted all or nothing. Without such a radicalist as head of state there would have been no WWII (in my opinion).

The other side of the coin might be interesting as well. What Hitler did was dangerous. He broke several treaties and Germany was in the early thirties so weak, that his provocations could have ended very soon in a disaster for him and his beloved country. So he was lucky to meet certain circumstances. So in my opinion chance plays a big role in the spectacle of history as well.

So presumed, there would have been no Hitler. I think, nevertheless there would have been a WWII. People were angry in those times in Germany because of high unemployment, the immovable situtation in politics and the Treaty of Versailles. I guess they would have voted for the communist party or another radical one. The Republic of Weimar did not broke down due to Hitler. There were many factors, all together they led to radicalism in one or another way. So there were tendencies. And a communist Germany plus a communist UdSSR.... I think, if there had been no WWII the Cold War would have turned out to a hot war.

Well... as always in history it could have happened completely different... who knows.....

So, what I wanted to tell is, that history is a flow of tendencies, crystallising into events, if there is the chance to do so. A single man will only be able to hold the reins, when history lets him do so.